U.S. patent number 5,751,703 [Application Number 08/645,005] was granted by the patent office on 1998-05-12 for energy dispersal method for tdma carrier.
This patent grant is currently assigned to Kokusai Denshin Denwa Co., Ltd.. Invention is credited to Takashi Inoue, Hiroyasu Ishikawa, Hideo Kobayashi.
United States Patent |
5,751,703 |
Kobayashi , et al. |
May 12, 1998 |
Energy dispersal method for TDMA carrier
Abstract
In a radio communication system using time division multiple
access system (TDMA) for access control method, modulation signal
sent by a slave station is frequency-dispersed during a TDMA burst
period by using a chirp signal which frequency changes continuously
on time axis so that signal power density on communication link is
decreased and interference signal affected by the slave station to
another radio communication system is reduced. According to the
present invention, modulation signal transmitted by each slave
station is multiplied to a chirp signal which frequency changes
continuously on time axis during a TDMA burst period so that
carrier frequency which carries the modulation signal changes
continuously on time axis to disperse on frequency axis to a
predetermined bandwidth thereby decreasing signal power density of
transmit signal from the slave station. In a master station,
frequency dispersed receive signal from the slave station is
multiplied to another chirp signal which has opposite
characteristics to that of the slave station during TDMA burst
period to obtain reproduced signal. Thus, interference signal by
the slave station to another radio communication system is
decreased, in a return link from the slave station to said master
station.
Inventors: |
Kobayashi; Hideo (Saitama,
JP), Inoue; Takashi (Tokyo, JP), Ishikawa;
Hiroyasu (Saitama, JP) |
Assignee: |
Kokusai Denshin Denwa Co., Ltd.
(Tokyo, JP)
|
Family
ID: |
15157061 |
Appl.
No.: |
08/645,005 |
Filed: |
May 9, 1996 |
Foreign Application Priority Data
|
|
|
|
|
May 11, 1995 [JP] |
|
|
7-135664 |
|
Current U.S.
Class: |
370/321;
370/442 |
Current CPC
Class: |
H04B
7/2643 (20130101); Y02D 30/70 (20200801); Y02D
70/22 (20180101); Y02D 70/449 (20180101) |
Current International
Class: |
H04B
7/26 (20060101); H04B 007/212 () |
Field of
Search: |
;370/321,322,337,347,348,442,468,315,316
;455/63,422,426,11.1,12.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Olms; Douglas W.
Assistant Examiner: Patel; Ajit
Attorney, Agent or Firm: Nikaido Marlmestein Murray &
Oram LLP
Claims
What is claimed is:
1. An energy dispersal method for a TDMA carrier in a radio
communication system comprising:
a plurality of slave stations, and
a master station which communicates with said slave stations
through a Time Division Multiple Access (TDMA) control system which
functions as a link control protocol,
each of said slave stations multiplying a modulation signal, which
is sent from said slave station by a first chirp signal, said first
chirp signal changes its frequency on a time axis during a TDMA
burst period, wherein said modulation signal is dispersed on a
frequency axis to a predetermined bandwidth by changing a carrier
frequency from said slave station to said master station during
said TDMA burst period, therein reducing signal power density of a
transmit signal of said slave station,
said master station a) receiving a frequency dispersed signal from
said slave station, b) multiplying said frequency dispersed signal
by a second chirp signal, which has opposite characteristics to
said first chirp signal which changes frequency continuously on
said time axis and which is used in said slave station during said
TDMA burst period, by using TDMA burst timing information, wherein
said modulation signal is reproduced in a frequency dispersed
received signal, and c) demodulating said reproduced modulation
signal by using a demodulator to provide a demodulated signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency dispersion
communication system for a modulation signal, and is used in all
radio communication systems including a digital station radio
communication system, a digital mobile radio communication system,
a digital satellite communication system, and a digital mobile
satellite communication system.
2. Prior Art
One of the current commercialized digital satellite communication
systems using a small earth station is a VSAT (Very Small Aperture
Terminal) system. Currently, a VSAT system is used in a private
network including a circuit exchange system and a packet exchange
system, but no direct access is carried out from a VSAT system to a
public network. That corresponds to the use of a satellite link
instead of a leased line which is used in a land communication
network.
The service by a VSAT system is classified into (1) a bidirectional
voice service and a middle rate data communication service, (2) a
bidirectional non-voice communication service including a packet
communication service, and (3) a one way image communication
service. The conventional network structures of typical VSAT
systems are classified into the following four services.
(1) Bidirectional Communication Service Using SCPC/FDMA
The main communication is voice, FAX, and data communication
(around 64 kbit/s), and a link control procedure is a
pre-assignment system, or a demand assignment system. As for a
network structure, a mesh structure or a star network structure is
possible. It is practically served in a private network between
companies for a local/international link. It is essentially a
leased network service having link exchange function for
point-to-point communication.
(2) Bidirectional Service Using TDM/TDMA
The main communication is packet type data communication which is a
non-voice communication, and the link control procedure is
reservation TDMA, or slotted aloha. It has been used in POS data
collection and stock management, credit card inquiry, satellite
LAN, ticket reservation, ATM, file transfer between computers. It
is essentially a private network having packet exchange function
for point-to-point service. The communication is mainly data
communication which is non-voice communication. Almost all the VSAT
networks in USA are in this category.
(3) One-way Communication Service
The communication has the tendency of broadcast communication such
as image etc., and generally, it is a one-way communication channel
from a central office to a VSAT. Some practical examples are
intracompany message, advertisement of products, lecture, seminar,
document, and distribution/broadcast service of voice and/or image.
An SNG service which sends image information from a VSAT to a
central earth station through one-way communication channel is in
this category.
(4) Integrated Communication Service
It is an integrated service based upon said three services (1)
through (3), and integrates them. A network in this service is
possible to cover low rate data communication, voice, FAX, high
rate image, and data communication.
As described above, various kind of services using a VSAT are
proposed, and some of them are commercially available, however, it
is merely the use as a private network, but a satellite
communication for the personal users has not been spreaded. One
reason why a VSAT has not been used widely is that it is difficult
to transport, trouble in usage, and mounting, because of a large
antenna with diameter of 1.2 m in a terminal station. Further, cost
for communication in a VSAT can not be reduced as compared with
that of a conventional land circuit.
In conventional consideration of a VSAT service considering the
above problems, it has been considered to be an auxiliary service
in specific area like a far or remote place, where no conventional
system is served. Therefore, available service has been restricted
to limited service like a private network, and so, no large amount
of users are expected, no efficient frequency utilization by demand
assignment is expected, and as a result, it falls into a vicious
cycle of high communication cost for communication.
On the other hand, a broadcast satellite (BS) antenna of
approximate 40 cm diameter has been popular with consumers because
of easy mounting, although it is only for reception. Therefore, an
USAT which has a smaller antenna than a VSAT has been proposed. An
object of an USAT is to provide a small size terminal in order to
improve easy mounting, transportation, and usage. A terminal with
an antenna in 40-50 cm of diameter in an USAT has a feature of
simple set up like a conventional BS antenna, it may be used in a
small building like a super-market, a bank, a restaurant etc., and
it may be used by consumers in a final stage.
In spite of the small size of the terminal equipment, communication
capacity does not decrease because of the latest improvement of
satellite power. The manner of service in an USAT is basically the
same as those in said VSAT. In other words, said three kinds of
services and the integrated service in a VSAT are available in an
USAT.
Further, the specific use of an USAT is as a portable terminal. The
use of a portable terminal differs from that of a VSAT which has a
semifixed terminal, but the service itself provided by an USAT is
the same as that of a VSAT. A direct access from an USAT terminal
to a public network, or from a public network to an USAT is not
used, but a private network is basically used. This differs from a
conventional Inmalsat system which has a portable terminal coupled
with a public network. Although the Inmalsat system provides only
voice service and low rate data communication because of
restriction of the allocation of the frequency band, an USAT system
which uses a fixed communication satellite having a wide frequency
band may provide high rate data communication, and image
communication etc. which a conventional mobile satellite
communication system can not provide.
In realizing an USAT system, a problem is interference with an
adjacent satellite system because of a small size antenna in an
USAT. No design specification in Ku band is established for an
antenna less than 1 m (50 wavelengths) of an aperture diameter.
Therefore, when an USAT system is realized, sidelobe
characteristics required for an USAT antenna based upon allowable
interference signal by an adjacent satellite must be clearly
defined, and an antenna which satisfies said characteristics must
be used.
However, even when an antenna which solves an interference problem
with an adjacent satellite is developed and is used, an
interference signal to an adjacent satellite would be a big problem
because of satellite tracking error due to an unpredictable
accident in an USAT system which is used by personal users. No
solution for that problem has been proposed, but an antenna which
satisfies sidelobe characteristics even considering the affect by
tracking error must be used. Therefore, an USAT antenna with a
diameter of 40-60 cm which is considerably smaller than that of a
VSAT antenna has been impossible in practice.
SUMMARY OF THE INVENTION
An object of the present invention is to solve an interference
problem to an adjacent satellite in an USAT system, and to provide
a frequency dispersion communication system which makes it possible
to use an antenna having a diameter of 40-60 cm in an USAT
system.
In order to achieve the object, according to the present modulation
signal frequency dispersed communication system, a modulation
signal sent by each USAT station in a group of USAT stations
multiplied by a chirp signal changes its frequency on a time axis
so that a carrier frequency of the modulation signal also changes
on a time axis to disperse to a predetermined bandwidth on a
frequency axis. Thus signal power density of a transmit signal of
the USAT station decreases. A center earth station (HUB station)
multiplies the frequency dispersed receive signal from the USAT
station with another chirp signal which has opposite
characteristics to that of the USAT station during a TDMA burst
period, to provide a reproduced signal. Thus, in a return link from
the USAT station to the HUB station, the interference signal, by
the USAT station, to another radio communication system is
decreased.
A chirp signal changes its frequency for instance continuously on a
time axis.
Because of frequency dispersion of a modulation signal from an USAT
station by using a chirp signal which changes its frequency on a
time axis, signal power density in a satellite circuit is
decreased, and transmit power density out of bandwidth of an USAT
station decreases. Thus, an interference problem to adjacent
satellites, due to a pointing error of an antenna, is solved. Then,
the size of an antenna used in a prior VSAT system having diameter
1.2 m is considerably decreased and a small size USAT antenna of
diameter 40-60 cm is possible, thereby, it may serve many uses
including personal users. Further, since a TDMA access control
system, used in a conventional VSAT system, is used in the present
invention as it is, the cost for construction of the present
invention may be low.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features, and attendant advantages
of the present invention will be appreciated as the same become
better understood by means of the following description and
accompanying drawings wherein;
FIG. 1 shows a block diagram of a modulation signal frequency
dispersed communication system using a chirp signal according to
the present invention, and
FIG. 2(a)-(f) show an embodiment of a frequency dispersion process
on a time axis in the modulation signal frequency dispersed
communication system using the chirp signal according to the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an embodiment of a circuit diagram of a modulation
signal frequency dispersed communication system which uses a chirp
signal according to the present invention. In the figure, an USAT
station 1 and a HUB station 2 communicates through a satellite 2.
The operation is described for a return link from the USAT station
1 to the HUB station 2. In the USAT station 1, a phase modulation
signal 11, provided by a phase modulator 10, is multiplied with a
chirp signal 16 provided by a chirp signal generator 13, so that
the phase modulation signal is converted to a frequency dispersed
signal 15.
As shown in FIG. 2, a chirp signal changes its frequency from
f.sub.1 to f.sub.2 on a time axis with a center frequency f.sub.0
of IF frequency of a communication channel during a burst period of
a TDMA signal. A frequency dispersed signal 15 which is product of
a chirp signal 16 and a phase modulation signal 11 is
frequency-converted to a radio frequency signal 18 by an up
converter 17, and is transmitted to a communication satellite 2
through an antenna 19. It is supposed that a chirp signal generator
13 and a phase modulator 10 are subject to burst timing
synchronization of a TDMA signal, so that a timing of a product of
a chirp signal 16 and a phase modulation signal 11, and a time
length of a product for a chirp signal are finely controlled by
using a burst timing information 12 which is provided by the phase
modulator 10.
In a HUB station 3, a frequency dispersed signal through a
communication satellite 2 is received through an antenna 20, and is
frequency-converted to a IF-band frequency dispersed signal by a
down converter 22, then, the IF-band frequency dispersed signal 23
is multiplied with a chirp signal 28 which is provided by a chirp
signal generator 29 and has opposite characteristics to that of the
chirp signal 16 in the USAT station 1 on a frequency axis. Thus, a
phase modulation signal 25 with a narrow bandwidth is obtained. The
phase modulation signal 25, which is inversely frequency dispersed
by using the chirp signal 28, is demodulated by a demodulator 26 so
that a demodulated information signal is obtained. A burst timing
signal 29, which is detected by using a burst signal series for
burst synchronization, controls the chirp signal generator 27, so
that the TDMA burst timing synchronization of the chirp signal
generator 27 and the frequency dispersed phase modulation signal 23
is finely established.
The above operation is shown by a spectrum on a frequency axis as
follows. The phase modulation signal spectrum 5, which is generated
by the phase modulator 10 in the USAT station 1, has a
predetermined bandwidth and a predetermined power density spectrum
around the center frequency f.sub.0 (4). When the phase modulation
signal 11 is multiplied with the chirp signal 16, the phase
modulation signal spectrum 5, which has a narrow band, is converted
to a frequency dispersed spectrum 8 in which the frequency spectrum
spreads from f.sub.1 (6) to f.sub.2 (7) around the center frequency
f.sub.0 (4) of the IF signal of a channel. The frequency dispersed
signal 8 is converted to a narrow band phase modulation signal 9 by
a anti-frequency dispersed operation through multiplication with
the chirp signal 28 which has opposite characteristics to that of
the chirp signal 16 on the frequency axis, in the HUB station
3.
FIG. 2 shows an embodiment of the operation of a frequency
dispersion process in FIG. 1 on a time axis. The symbols (a)-(c) in
FIG. 2 correspond to the phase modulation signal 11, the chirp
signal 16 and the frequency dispersed signal 15, respectively, in
the USAT station in FIG. 1, and also correspond to the frequency
dispersed received signal 23, the chirp signal 28 and the phase
modulation received signal 25, respectively, in the HUB station in
FIG. 1.
In FIG. 2(a), the phase modulation signal 41 appears during the
whole burst period T.sub.b, and a guard time T.sub.g (42), for a
timing control error, is inserted between adjacent burst signals.
The length of a burst period of a TDMA signal is defined as T.sub.b
+T.sub.g. The phase modulation signal 41 is multiplied with the
chirp signal 46 in FIG. 2(b), which has its frequency change
continuously from f.sub.1 (44) to f.sub.2 (45) with a center
frequency f.sub.0 (43), then, the product of the multiplication is
a frequency dispersed signal 49 in FIG. 2(c). It should be noted
that the multiplication of the phase modulation signal 41 with the
chirp signal 46 is carried out only during the signal transmission
period T.sub.b (40), and the frequency f.sub.2 (47) of the chirp
signal is reset to f.sub.1 (44) during the guard time (47).
In FIG. 2(d), the frequency dispersed signal 50 received in the HUB
station is divided into a TDMA burst signal with the time length
T.sub.g (48) based upon a TDMA burst timing information (not
shown). Since a guard time T.sub.g (42) is set between each burst
signals, the frequency dispersed signal (50) having a time length
T.sub.b (40) is always within the burst length T.sub.s (48) of a
TDMA signal.
The frequency dispersed signal 50 received in the HUB station is,
then, multiplied with the chirp signal 54 of FIG. 2(e) which has
opposite characteristics on a frequency axis to that of FIG. 2(b),
and the phase modulation signal in FIG. 2(f) is obtained. The
instantaneous frequency of the chirp signal 54 changes continuously
on a time axis from f.sub.2 '(52) to f.sub.1 '(53) during a TDMA
burst period T.sub.s (48) from top to burst end. Although the chirp
signal 46 in FIG. 2(b) changes continuously on a time axis between
instantaneous frequencies f.sub.1 (44)-f.sub.2 (45), since the HUB
station takes timing control so that a burst timing control error
is absorbed in the TDMA guard time T.sub.g (55), the frequency band
f.sub.1 (44)-f.sub.2 (45) is always included in the frequency band
f.sub.1 '(52)-f.sub.2 '(53).
If a TDMA burst timing control has an error, a phase modulation
signal 56 after anti-dispersion has some frequency offset, however,
the affection by the offset may be simply removed by using a
conventional frequency control circuit (AFC).
Although the above embodiment is directed to a digital
communication system for a fixed type station using a satellite,
the present invention is applicable to a mobile communication
system using a satellite, merely by replacing a USAT station with a
mobile station, and that system is obvious to those skilled in the
art based upon the embodiment described.
The present invention is applicable to a land mobile communication
system by substituting a mobile station in a land mobile
communication system with the USAT station, and a base station and
a network control station in the land mobile communication system
with the HUB station. Further, the present invention is applicable
to any kind of radio communication system including a land
communication system using a fixed type radio terminal.
Although a phase modulation technique which is usually used a in
satellite communication system is described in the embodiment, the
present invention is applicable to any modulation and/or
demodulation system.
Further, although a frequency of a chirp signal which changes
continuously on a time axis in each TDMA burst period, is linearly
changed in the embodiment, the frequency change ratio is not
restricted to a linear function, but a wavelet function, a
rectangular function, and other functions are possible.
The present invention has at least the following effects.
(1) As signal power density in a return link from a slave station
to a master station is decreased, an interference signal power to
another radio communication system is reduced.
(2) Since signal power density in a return link from a slave
station to a master station is decreased, and since an interference
signal power to another radio communication system is reduced, it
is possible to use a small simple antenna which has a broad beam
pattern and is simple in design, and therefore, cost for an antenna
system in a slave station is reduced.
(3) A network control system in a conventional radio communication
system is used in the present invention. So, the cost for
constructing a system of the present invention is low.
(4) As for a spurious impulse signal on a frequency axis generated
in a transmitter in a slave station, the spectrum of the spurious
signal is dispersed by anti-dispersion in a receiver in a master
station. Therefore, the affect on a desired modulation signal by
the spurious signal is reduced.
From the foregoing it has now been apparent that a new and improved
energy dispersal method for a TDMA carrier has been found. It
should be appreciated of course that the embodiments disclosed are
merely illustrative and are not intended to limit the scope of the
invention. Reference should be made to the appended claims,
therefore, for indicating the scope of the invention.
* * * * *